A method is proposed to estimate the number of vibration cycles caused by frequency locked-in ice loads on a bottom fixed structure. Studies of the onset of frequency lock-in have been performed and based on that, different onset criteria are studied. It is shown in this work that most existing onset criteria significantly overestimate the vulnerability to frequency locked-in ice loads. The method described proposes a 90 % reduction in the number of vibration cycles in a winter season. The 2002–2003 winter at Norströmsgrund lighthouse is compared with the proposed method. The comparisons show that all available methods overestimate the number of vibration cycles significantly.

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